<html>
  <body>
    <p id="out"></p>
    <script>
      let input = prompt("请输入运算符号");
      let output = document.getElementById("out");
      output.innerHTML = "你输入的";
      switch (input) {
        case "+":
          output.innerHTML += "+";
          break;
        case "-":
          output.innerHTML += "-";
          break;
        default:
          output.innerHTML += "符号无法处理";
      }
    </script>
  </body>
</html>
